The present invention relates to sensors for composites. More particularly, this invention relates to methods and devices for sensing and predicting strain and/or damage in composites.
New generations of structures, such as, but not limited to, bridges, buildings, vehicles, boats, turbine blades, wind turbine blades, space vehicles and aircraft, are being built with larger amounts of composites in primary structures, thus creating a need for new testing and monitoring systems to prevent failures, from minor structural failures to catastrophic failures. The use of composites creates new risks, because an unseen flaw could render a structure unsafe, and yet determining the condition within a composite is difficult for structural engineers and inspectors.
Failure mechanisms common to composites, such as micro-cracks, delamination and fiber pull-out, are different from those seen in metals typically used as structural materials. For example, a severe impact may leave a visible dent in a metal sheet, but may result in little surface damage in a composite. Despite the small amount of surface damage in the composite, the impact may have caused microcracking and backface delamination. Unless detected, such a defect may propagate when the composite is placed under load, resulting in catastrophic failure. For the incorporation of composites into large structural components, little information is available on the mechanisms of aging and property degradation that controls service life compared to metals.
Additionally, the rates and mechanisms of long term property degradation during regular service are poorly understood in composite structures. Moreover, monitoring defects and damage is difficult in thick composite laminates resulting in catastrophic failures that may occur without warning.
Structural health monitoring is a process of implementing a damage sensing strategy which is expected to prevent catastrophic failure by, partially or completely, predicting the onset of failure. Typically, damage may be referred to as an adverse change in a material's properties that can affect the current or future performance of the material. Damage eventually leads to failure of the structure, the condition when it is no longer acceptable for normal use. The need for monitoring the integrity of a structure becomes more critical for components having brittle behavior. Structural health monitoring may be used as a surveillance technique to monitor damage initiation and progression, and thus allowing for remedial action and prevention of catastrophic failures. Embedded or surface-mounted real time structural sensor systems will ultimately lead to reduced maintenance and operational costs and allow working with lower design safety tolerances. Early damage detection would prevent system downtime and associated costs.
The concept of structural health monitoring is applicable to civil, aerospace, and mechanical component industries. Monitoring a component built from an isotropic material, however, is considerably simpler than composite structures that are quasi-isotropic in nature. Furthermore, in addition to different failure mechanisms compared with metal constructions, composites may contain invisible microcracks and delaminations. Such defects propagate under load. Being a relatively new system, very limited information is available on the mechanisms of aging and property degradation in the composites, which controls the service life. The sensing technique, therefore, must be rugged and must be able to withstand severe operating conditions. These conditions include aerodynamic stresses, prolonged exposure to extreme temperatures, ultraviolet (UV) radiations and humidity. The sensor must be able to identify the repetitive changes in strain as well as sudden deformations (damage, crack, delamination).